Resolvers are used extensively in motor control systems to provide angular position and velocity feedback. Generally, the conventional resolver includes a rotating primary winding energized from a carrier or reference frequency source via slip rings or the like. The conventional resolver also includes a pair of stationary secondary windings positioned in a quadrature relationship. The secondary windings provide signals at the carrier frequency with an amplitude envelope corresponding to sine .theta. and cosine .theta., respectively, where .theta. is the angular position of the resolver shaft. These signals are then demodulated to remove the carrier and recover the amplitude envelope. The sine .theta. and cosine .theta. signal values are then processed to determine the angle .theta. which is the shaft position. The advantages of the resolver over other types of position indicators are relatively low cost and virtually unlimited resolution provided by the continuous analog signals.
A known alternative to the conventional resolver is the variable reluctance resolver. Such resolvers do not have any rotating windings and therefore eliminate the need for slip rings or other forms of sliding contacts. The rotor structure is a relatively simple laminated iron salient pole structure without any deep slots which would otherwise be required to hold the rotating windings. The primary winding is on the stator and is energized from the carrier source. The secondary windings in a quadrature relationship are also on the stator. The energy coupled form the primary to the secondary windings depends on the reluctance of the coupling and is a function of the rotor position. Because of the simple rotor structure, variable reluctance resolvers are less expensive to construct. The variable reluctance resolvers, however, do not produce clean sine and cosine signals having a known relationship to shaft position and, as a result, are not usable in most servo applications where a high degree of linearity is required.